The present invention relates to a spindle servo unit for controlling the rotational speed of a recording disk in a disk playing device for playing video disks or the like.
Such disk playing devices ordinarily are constituted so as to provide a coarse adjustment to a time base by controlling the relative velocity between a disk and a pickup by means of a spindle servo unit which controls, the rotational speed of the disk. These devices eliminate residual jitter due to eccentricity of the disk and the like which cannot be removed by means of the coarse adjustment alone, by providing a fine adjustment to the time base by delaying a read signal read from the disk with the pickup by a length of time corresponding to the phase difference between a synchronous signal in the read signal and a reference signal generated separately by using a CCD. memory or the like.
A disk playing device having a conventional spindle servo unit is shown in FIG. 5. In the Figure, a recording disk which has video signals recorded thereon is rotated by a spindle motor 2. The recorded signals are read by a pickup 3 which is positioned with respect to the recording disk 1 so as to trace desired tracks by means of a tracking servo mechanism 4. For a special reproduction mode such as still reproduction, doublespeed reproduction and the like, the tracking servo mechanism 4 causes a read spot (information detecting point) of the pickup 3 to jump from one track of the recording disk 1 to another track which is more than one track away.
A recorded signal detected by the pickup 3 is converted to a video signal by being demodulated by a demodulation circuit 5 consisting of an FM demodulator or the like. The output of the demodulation circuit 5 is supplied to a horizontal synchronization detection circuit 7 composed of a synchronization separation circuit or the like as a reproduced video signal. The jitter correction circuit 6 consists, for example, of an A/D (analog to digital) converter which A/D converts the reproduced video signal and a FIFO (first in--first out) memory to which is supplied the output of the A/D converter. In the jitter correction circuit 6, the reproduced video signal is converted to digital data in accordance with a reproduced horizontal synchronizing signal which also is separately detected in the reproduced video signal by the horizontal synchronization detection circuit 7. The circuit 6 performs fine adjustment of the time base by writing the reproduced signal in the FIFO memory and then reading and outputting the data written in the FIFO memory using a reference signal which is generated separately. The output of the jitter correction circuit 6 is supplied to a video output terminal.
Further, a reproduced horizontal synchronizing signal output by the horizontal synchronization detection circuit 7 is supplied to a phase comparison circuit 8. In the phase comparison circuit 8, respective phases of the reproduced horizontal synchronizing signal and a reference signal output from a reference signal generating circuit 9 consisting of a crystal oscillator and the like are compared, and an error signal corresponding to the difference between the phases of the two signals is produced. The output of the phase comparison circuit 8 is supplied as a drive signal to the spindle motor 2 via a servo amplifier 10. As a result, coarse adjustment of the time base is achieved by controlling the rotational speed of the spindle motor 2 so as to maintain the phase difference between the reproduced horizontal synchronizing signal and the reference signal.
Now, when a track jumping operation is executed while playing a CLV (constant linear velocity) disk using the conventional spindle servo unit as described above, output of the phase comparison circuit 8 is disturbed due to the fact that the portions of the CLV disk on which horizontal synchronizing signals are recorded are not aligned radially on the disk. As a result, there is a failure in normal phase control of the spindle motor and a large error in the time base of the reproduced video signal causing disturbance of the reproduced picture.